1. Field of the Invention
The present invention relates to a laser system capable of emitting two or more coherent wavelengths of light, and more particularly to both optically pumped and electrically pumped semiconductor vertical external cavity surface emitting lasers (VECSEL) with selectively positioned quantum wells to control the power of light at each of said wavelengths.
2. Description of the Related Art
There is an increasing need for multiple wavelength laser sources. For instance, the increased popularity of laser display devices presents a need for a red (R)(˜625 nm), green (G)(˜532 nm) and blue (B)(˜460 nm) laser source. Currently, the present inventors know of no available semiconductor lasers that emit both blue and green wavelengths.
Diode pumped solid state (DPSS) lasers combined with second harmonic generation (SHG) technology can provide green laser light at 532 nm, but such devices are relatively expensive at this time. High power blue laser light is still very difficult to achieve, due to low recombination efficiency.
Infrared wavelength, optically pumped semiconductor (OPS) vertical external (or coupled) cavity surface emitting lasers (VECSEL), such as disclosed in U.S. Pat. No. 6,347,109, can provide a visual wavelength laser beam with SHG technology, such as disclosed in U.S. Pat. No. 6,370,168. Similarly, electrically pumped VECSELs, such as disclosed in U.S. Pat. Nos. 6,614,827 and 6,243,407, can provide a visual wavelength laser beam with SHG technology. Each of these patents is herein incorporated by reference.
Generally, OPS-VECSELs conceptually combine the approaches of diode-pumped solid-state lasers and semiconductor quantum-well (QW) vertical-cavity surface-emitting lasers (VCSEL). In these approaches, a semiconductor chip of the laser system is composed of a QW active layer and a highly reflective distributed Bragg reflector (DBR). Usually the QW active layer is designed to enhance gain performance to achieve high power, such as provided by a resonant periodic gain (RPG) structure, such as disclosed by Mark Kuznetsov et al., “Design and Characteristics of High-Power (>0.5−W CW) Diode-Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Lasers with Circular TEM00 Beams,” IEEE J. of Selected Topics in Quantum Electronics, Vol. 5, No. 3, May/June 1999.
FIG. 1 is a simplified cross-section of a conventional VECSEL, such as disclosed in U.S. Pat. No. 6,370,168. Referring to FIG. 1, the conventional VECSEL 10 includes a heat sink 11 attached to a chip. The chip includes a substrate 12 upon which are stacked a lower, high reflectivity multi-layer mirror 13, such as a semiconductor layer of distributed Bragg reflectors (DBR), a gain region 14, such as a multi-quantum well gain region with a resonant periodic gain (RPG), and an upper anti-reflective coating 15. A lasing cavity is formed between an external spherical mirror 16, through which part of the laser beam λ2 can pass as the laser output, and the high-reflectivity mirror 13 of the chip. A pump beam from a multi-mode laser source 17 of a different wavelength λ1 is projected onto the anti-reflective coating 15. An optical non-linear crystal (not shown) can be added to double the frequency of the lasing light λ2.
Referring to FIG. 2, the structure of the conventional DBR 13 and RPG active region 14 is shown in relation to the standing wave of the cavity structure. As can be seen, the RPG 14 is made up of QWs 14a and spacers 14b. The QWs 14a are positioned at the antinodes of the standing wave to maximize gain in the pumped laser light λ2 through in-quantum-well absorption of the pumping laser light λ1, which is favored to avoid excessive heat generation. Light generated from this active region 14 of a conventional VECSEL is projected through the external mirror 16 as a continuous wave (CW) laser light output.
In approaches such as these, the pump diode laser 17 (in the case of optical pumping), the cooling element, and the optics determine the laser system price. The semiconductor chip (the substrate 12, the DBR 13, the RPG 14 and the anti-reflective coating 15) represents a small fraction of the total cost. These systems generate a single wavelength λ2 of laser light as the output.